This invention relates to a guidepost system used to aid in aligning an object lowered from the ocean surface to a predetermined location on a subsea structure. More particularly, the invention pertains to a method and apparatus for remotely replacing damaged guideposts attached to subsea structures.
The exploitation of offshore oil fields frequently involves the construction of fixed subsea facilities for use in conducting drilling, completion and production operations. Fixed subsea facilities are especially useful in oil fields located in deep water where conducting such operations from fixed or floating surface structures would be either prohibitively expensive or technically unfeasible. The installation, operation and maintenance of subsea facilities requires that various types of equipment be lowered from the surface of the body of water to a precise location on the subsea structure. In order to accomplish this, means must be provided for properly aligning the lowered equipment with the target area on the structure.
The most widely employed method of accomplishing this alignment is by the use of guidelines. In a typical guideline system a base is mounted on the subsea structure at the target location. One or more upright guideposts are attached to the base. A tensioned wire rope guideline is connected to the top of each guidepost and extends upwardly to the surface of the body of water. The equipment being lowered is attached to a guide frame which is lowered down the guidelines until it engages the guideposts. In this manner the equipment is directed to the desired position on the subsea structure. See, for example, the guideline system disclosed in U.S. Pat. No. 3,050,139 issued to Hayes (1962).
Guideposts may also be used to accomplish the necessary alignment without attached guidelines. See, for example, the alignment system described in U.S. Pat. No. 3,545,539 issued to Manning (1970).
Guideposts attached to subsea structures are subject to damage which oftentimes necessitates replacement. Damage to the guideposts may occur in several ways. For example, undersea currents can cause heavy equipment being lowered to the subsea structure to shift, thereby striking and bending the guideposts. Alternatively, the equipment being lowered may shift or drop suddenly due to wave induced movement of the surface vessel or structure. This sudden movement may damage or destroy the guideposts.
At best, replacement of a damaged guidepost is extremely difficult. Typically, guideposts are either welded or bolted to the subsea structure. If the subsea structure is located in shallow water, divers may be used to remove the damaged guidepost and install an undamaged one in its place. If, however, the subsea structure is located in deep water, a mini-submarine or other manned submersible vessel must be used to make the replacement. Thus, there is a need for a guidepost system which permits damaged guideposts to be removed and replaced with undamaged ones from a remote location, such as the surface of the body of water.